Question

how us humidity measure.

Answer 1

Often scaled in units of dew point (or frost point) – hence the instrument name – but readings can be displayed in other terms. Psychrometer (wet- and dry-bulb hygrometer) - Hygrometer using evaporative cooling as a measure of humidity. A dry thermometer is compared against one sheathed in a wet wick, in moving air.

Answer 2

hygrometer is used to measure humidity.